The present invention relates to a method and a device for adjusting the braking action in a motor vehicle.
Many conventional possibilities exist for counteracting the locking tendency of a braked wheel by adjusting the braking force at each wheel, for example along the lines of an individual control, such that a predetermined brake slip is maintained. As a result, a limitation on the braking torque is achieved. Furthermore, it is conventional to counteract the locking tendency of a braked wheel in that, in the context of an instability control, an instability value is generated at least as a function of the brake slip and of the wheel deceleration, and this instability value is compared with a predefinable threshold value.
If a vehicle that has a brake system of this type is braked on a roadway that produces on the one side of the vehicle a higher coefficient of friction and on the other side of the vehicle a lower coefficient of friction, then the different braking forces can produce yawing torques. In order to avoid or reduce this effect, a multitude of conventional possibilities exist to reduce or delay the buildup of yawing torques.
The aforementioned condition, that on the two sides of the vehicle significantly different coefficient of friction values are operative, is termed the xcexc-split condition. When brakes are applied in a xcexc-split condition of this type, different braking forces are operative at the front wheels of the vehicle, In accordance with the different coefficients of friction between the left and right sides of the vehicle, the different braking forces cause a torque about the vehicle vertical axis, i.e., the yawing torque. The problem is the magnitude and the rapid increase of the yawing torque. By delaying the buildup of the yawing moment, the buildup of the brake pressure is delayed on the side of the vehicle that has the high coefficient of friction, as a result of which the driver is given the opportunity to counteract the now slower buildup of the yawing torque by a corresponding steering angle correction. By way of example, the select-low control at the rear axle of the vehicle is conventional, and with which both rear wheels are controlled in accordance with the unstable rear wheel, i.e., the wheel on the side that has a lower coefficient of friction. As a result, the rear wheel on the side having the high coefficient of friction is significantly underbraked and can be operated as a Peissler wheel (a free-running wheel) for determining the vehicle longitudinal velocity, the vehicle reference velocity. Therefore, the velocity reference is based on this quasi free-running wheel.
In vehicles that have a large load differential at the rear axle, for example in the case of commercial vehicles, the select-low control at the rear axle implies a relatively long braking distance. Therefore, in vehicles of this type, individual control is applied at the front and rear axles. This means that the braking action on each individual wheel of the vehicle is adjusted independently of the dynamic conditions at the other wheels. In this context, it is provided that at least the wheels on the side that has the high coefficient of friction are set at a predetermined brake slip. To determine the brake slip, however, a stable reference variable in the form of a dependable value for the vehicle reference velocity is provided.
An object of the present invention is to provide a stable reference variable for delaying the buildup of the yawing torque by adjusting the braking action at the wheels, without having to accept unnecessarily long braking distances.
The present invention relates to a method and a device for adjusting the braking action at the wheels of a motor vehicle that has at least one rear axle and one front axle. For adjusting the braking action, the brake systems of the wheels are driven in a corresponding manner. Rotational speed quantities representing the rotational motions of the wheels are measured. As a function of these measured rotational speed quantities, drive signals are determined for driving the wheel brakes at least along the lines of an increase or reduction in the braking action. Furthermore, at least as a function of the measured rotational speed quantities, as a xcexc-split situation, the situation is detected in which there are distinctly different coefficients of friction at the right and left wheels of the vehicle. As was mentioned, the wheels that are located on the roadway that has the lower coefficient of friction are designated as low wheels, whereas the wheels that are located on the side of the roadway that has the higher coefficient of friction are termed high wheels.
According to the present invention, in the event of a xcexc-split situation the brake systems of the low wheel at the rear axle are driven such that the reduction of the braking action at the low wheel at the rear axle is brought about as a function of the reduction of the braking action at the same-side wheel at the front axle. As an alternative to this, it is provided that essentially no braking action is carried out at the low wheel at the rear axle.
According to a first embodiment of the present invention, the reduction in the braking action, in general the reduction in rake pressure, at the low front wheel is copied to the same-side low rear wheel. As a result, the low rear wheel, which is on the side that has the lower coefficient of friction and which in general does not significantly contribute to the braking power, i.e., to the vehicle deceleration, is underbraked and therefore, can function as reference variable, i.e., as the reference velocity.
According to a second embodiment of the present invention, at first no significant braking action is applied at the low rear wheel. In this way as well, the low rear wheel on the side that has the lower coefficient of friction can aid in supporting the vehicle reference velocity. The second embodiment of the present invention, however, has the disadvantage, in comparison to the first embodiment of the present invention, that when the coefficient of friction is changed on the low side, the low wheel at the rear axle in the first embodiment contributes to the deceleration of the vehicle, whereas the low wheel at the rear axle in accordance with the second embodiment remains without braking action.
In an advantageous embodiment of the present invention, it is provided that in the event of a xcexc-split situation, the brake systems at the high wheels of the front and rear axles are driven such that a preselected brake slip is applied. In this so-called individual control, it is provided that the brake slip is calculated as a function of the rotational speed quantities, and the brake slip is compared to a threshold value. The driving of the brake system of the high wheels is then performed as a function of the comparison.
In the latter embodiment, the present invention provides that in order to adjust the brake slip, at least the brake slip at the high wheels of the front and rear axles is measured as a function of the rotational speed quantity representing the rotational motions of the low wheel at the rear axle. In this context, is provided that the rotational speed quantity representing the rotational motions of the low wheel at the rear axle is used as the vehicle velocity, or vehicle reference velocity, in calculating the brake slip.
In general, in passenger cars, an individual control is selected for the front axle and a select-low control is chosen for the rear axle. In contrast, in trucks, a select-low control is provided at the front axle and an individual control is provided at the rear axle. Individual control for rear and front axles results in shorter braking distances in particular in light commercial vehicles and in response to braking actions in a xcexc-split situation, since these vehicles are classified according to weight distribution between a passenger car and a truck.
According to the present invention, as a result of the generation of a reference variable in accordance with the aforementioned principle for the slip-controlled delay in the buildup of yawing torque, it is therefore possible to additionally assure stability for a vehicle throughout the entire braking process.
It is advantageous that in the event of a xcexc-split situation, the brake systems of the low wheel at the front axle are driven so as to prevent this wheel from locking. In this context, as was mentioned above, it is especially provided that an instability value representing the dynamic of this wheel is calculated at least as a function of the rotational speed quantities. This instability value is compared with a threshold value, whereupon the brake system of this wheel is driven as a function of the comparison.
The adjustment of the braking action can be brought about by adjusting a hydraulic braking pressure. In this case, the increase and the decrease of the braking action is achieved as a result of the fact that, through a corresponding driving of the brake pressure valves, the brake pressure in the brake systems of the wheels is increased or decreased. In addition, of course, provision can also be made to maintain the brake pressure at a constant level. In this context, it is provided that the driving processes represent the duration of a brake pressure buildup and a brake pressure builddown.
If a xcexc-split situation is detected, the brake systems of the low wheel at the rear axle are driven, according to the present invention, such that the duration of the brake pressure builddown at this wheel is actuated as a function of the duration of the brake pressure builddown at the same-side front wheel.